1. Field of the Invention
The present invention relates to measuring the lateral displacement and/or the lateral dimension and changes in its lateral dimension of an object, particularly a moving filament or wire, from its nominal path. The term "filament" is used generically hereinafter with respect to both metal and non-metallic strands.
2. Description of the Prior Art
The manufacture of metal wire and glass or plastic filaments involves some common steps, namely the drawing of the filament. Metal wire is usually drawn from a billet through several dies, and the path is generally horizontal to the take-up. Glass filaments or filaments of synthetic materials are usually drawn vertically downward to a take-up. Either orientation of the forming filament should be accommodated by measuring apparatus. Such drawing operations operate at high speed, and are closely controlled so as to produce filaments of uniform density and diameter, and it is most important to have an accurate means of measuring the lateral dimension or diameter of the filament without contacting the material being drawn. By way of example, filament speeds in glass fiber manufacturing can be in the order of 5,000 meters/minute, and in wire manufacturing can be in the order of 1,000 meters/minute.
It is also important to recognize and identify any displacement of the filament from its designated path of movement at these high speeds. The diameter of the filament can be measured (by techniques disclosed hereinafter, or by other techniques), or the diameter can be estimated as a known value. Vibration in the length of the moving (running) filament will result in related waves of corresponding frequency in the filament length. Knowing the mass per unit length of the filament, by determining the displacement due to such vibration, it is possible to calculate the tension in the filament, on an ongoing basis. This ability has a number of advantages for process control purposes.
Many different techniques have been proposed for measuring filament dimension and/or displacement. These techniques fall generally into two categories: a) scanning devices and b) non-scanning devices. Scanning devices further are divided into two categories: 1) scanned light source, and 2) scanned sensing of light.
In the case of category a1) the scanned light is mechanically or optically moved laterally past the object; see for example U.S. Pat. No. 3,765,774 issued 16 Oct. 1973. The time that the light is not present at the receiver is proportional to the lateral dimension of the object. In category a-2), the light source is stationary and the receiver is scanned laterally across the object. Both scanning cases have the advantage of simplicity and the disadvantages of scan-to-scan variation and limitations on the scanning rate, thus limiting the number of samples of the object and subsequent failure to measure every portion of the object, particularly if it is in motion past the scanning device at a speed approximately greater than the width of the scanning light beam (for category a-1) or the width of the field of view for the receiver (for category a-2) multiplied by the number of scans per unit time.
With regard to non-scanning devices, a light beam is generally produced that is uniform in amplitude or brightness across the beam, is usually collimated so as to produce a nearly parallel beam. Interposing an object in such beam blocks off an amount of light that is proportional to the diameter of the object. The advantage of this technique is that continuous measurement of the lateral dimension of the object is possible. The drawbacks are that the apparatus requires very uniform illumination of the object, sensitivity to background or stray light, inability to measure lateral position, and sensitivity to gradual changes in the lateral dimension of the object to be measured.